Although there are a number of established techniques for providing high resolution images of the internal structure of large single crystals, as is discussed below, these techniques are not suitable for verification of the grain flow in a curved or a convex surface of a polycrystalline sample, such as the curved surface of an article made of aluminum. In such an article, the grains of the curved surface of the formed article are flattened during the forming process and are elongated in one direction, with the relatively large faces thereof oriented parallel to the curved surface. It is noted that rotation of these grains about the longitudinal axis into a position perpendicular to the curved surface would make imaging of the grains easier to accomplish but such an approach is undesirable because of the resulting changes in the formed article so produced. On the other hand, the flattened elongated grains of the formed article present special problems insofar as providing high resolution imaging of the grains is concerned.
As mentioned above, there are established methods for providing high resolution images of large single crystals and in this regard, x-ray topography techniques have been used for several years to provide high resolution images of the internal structures of such large single crystals. These methods are used to produce a point-by-point correspondence between the incident x-rays striking the surface and the diffracted x-rays striking a film. Such x-ray topography methods are described, for example, by B. K. Tanner, in "X-Ray Diffraction Topography," Pergamon Press, New York, N.Y. (1976). A further reference in this field which describes x-ray metallography techniques of interest, including the Berg-Barrett method discussed below, is A. Taylor, "X-Ray Metallography," John Wiley & Sons, Inc., New York, London (1961). The Berg-Barrett method just referred to provides for locating the sample a long distance from the x-ray source so that the x-ray beam will appear to be neady parallel, and then placing the film very close to the sample surface to limit the divergence of the diffracted beam. A Berg-Barrett method has been used for producing images of grains in polycrystalline uranium. This method is described by L. Le Naour, in "X-Ray Topography of Uranium Alloys, ORNL-tr-5069, (translated from the French CEA Report, CEA-R-3494), Union Carbide Corporation, Nuclear Division, Oak Ridge National Laboratory (May 1968). Methods based on crossed-sollor slits for limiting the divergence of the diffracted beam have produced images which show texture variations in rolled aluminum samples. These methods have been described by Y. Chikauro, Y-Yoneda and G. Hiidebrant, in "Polycrystal Scattering Topography," J. Appl. Cryst., 15, 48 (1982).
In each of these methods, parallel and/or divergent x-ray beams are directed toward samples with flat surfaces for diffraction thereby, and these methods can not be used efficiently for imaging grains in a curved polycrystalline surface.